US3116390A

US3116390A - Dual element fuses

Info

Publication number: US3116390A
Application number: US55361A
Authority: US
Inventors: Robert B Goody; Kafenbaum Wiliam
Original assignee: Federal Pacific Electric Co
Current assignee: Federal Pacific Electric Co
Priority date: 1960-09-12
Filing date: 1960-09-12
Publication date: 1963-12-31
Anticipated expiration: 1980-12-31

Description

1963 R. B. GOODY ETAL 3,116,390

' DUAL ELEMENT FUSES Filed Sept. 12, 1960 FIG.2

FIG. 5 7 FIG. 6

6 I I I 1 24 14 22 24 EZZHII i lll/ "*716 51; A [m INVENTORS ROBERT B. GOODY WILLIAM KAFENBAUM ATTORNEY United States Patent Ofiice Patented Dec.

3,1163% DUAL ELlEh/IENT FUSES;

Robert ll. Goody, Tenaily, and William Kafenhaum, Metuchen, NJ assignors to Federal Pacific Electric Company, a corporation of Delaware Filed Slept. 12, 1195s, Ser- No. 55,361 3 'll. Ztlll l) The present invention relates to electrical fuses and to fuse links.

Electrical fuses have the characteristic of blowing and interrupting the protected circuit almost instantly in response to severe overloads and, after a desired delay time, in response to moderate overloads. it has been considered desirable that for overloads of only 10%, for example, the fuse should remain conductive for an almost indefinite time while, after flow of 135% of rated current for some limited period of time, perhaps an hour, it has been required that the fuse should blow and clear the circuit. An object of this invention is to provide a novel fuse that will blow and clear the circuit within a shorter period of time in response to substantial overload, while holding in during slight current overload conditions.

The time when this occurs is a matter of considerable uncertainty unless some trigger arrangement is ntil zed to cause sudden blowing of the fuse when subjected to a moderate overload for a substantial but reasonably consistent interval of time.

Accordingly an object of the present provide an improved fuse having a novel a for causing the fuse to blow in response to 0v load of moderate proportions after a standardized period of overload current. A related obiect resides in the provision of a novel fuse characte to maintain current flow in a circuit having a small overload current and to clear he circuit after a specified rim interval at a specified level of moderate overload with excellent consistency from one fuse to others of the same rating.

The nature of the invention and its various features of novelty and further obg'ects will be apparent from the following detailed disclosure of an illustrative embodiment of the invention. this embodiment a fuse link of silver or other metal having a relatively high melting temperature and high electrical conductivity is provided with apertures a body of low melting tern pcrature alloy, such as solder, having a central bore therein, is then formed in one of the apertures. The fuse link and its lowrnelting alloy body overheat in response to overcurrent conditions in the load circuit; and when the temperature is sufficiently high, the alloy llovs. The central bore enlarges when the solder melts and flows outwardly along the link. The cross-section of electrical and thermal conductor recresented by the iolten alloy at the aperture decreases, and the local rise in temperature accelerates. The rise temperature also induces alloying of. the alloy body i ;o the fuse link, resuiti g in increased electrical rcsisuvity and fur the acceleration in the heating, all of which loans to a sudden rupture of the fuse.

it is accordingly seen that the principal ooject of the invention resides in the provision of a novel electrical fuse element having improved time-overcurrent characteristics.

The illustrative embodiment of the invention is shown in the accompanying drawings forming a part of the present disclosure. in the drawings:

FIG. 1 is a longitudinal cross-section of a fuse embodying the present invention, viewing the fuse link edgewise;

invention is to trigger mech FIG. 2 is a plan view of the fuse link in the embodiment of FIG. 1;

FIG. 3 is an enlarged cross-section of a portion of the fuse link in FIG. 2 along the line 3--3 therein;

FIG. 4 is a transverse cross-section viewed from the line of FIG. 3;

FIG. 5 is a view corresponding to FIG. 3 in a phase of fuse operation which is near to the long-time over-current interruption; and

FIG. 6 is a transverse cross-section viewed from the line 6@ in FIG. 5.

Referring now to the drawings in which the relative proportions are substantially to scale, a cartridge fuse of typical construction is shown, including brass end caps lid and if, a tube 12 of suitable insulation such as fiber or the like, and a fuse link 14 soldered at its ends in good thermal and electrical contact to the end caps and which serve as terminals. The usual filling of sand (not sh wn) is, of course, contemplated. Fuse link 3% includes a series of apertures 16, fit and Ztl formed in a thin strip of relatively high-melting-temperature metal such as silver, this also being a metal of low resistivity so that the fuse link operates at low temperatures for normal currents. Other high-mslting-temperature metals such as copper might also be employed in the production of the link 114. Silver is preferred be cause of its superior thermal and electrical conductivity relative to solder.

in one method of producing the subject link, a closure is formed over the aperture 26 in the form of a solder window 2 2 by wiping molten solder over an aperture. This may be accom; for example, by supporting the i e link on a solder-repellent surface and wiping a soldering iron across the fuse link. in this operation, the fuse 1i 7 is heated sufficiently to cause tinning, that is, to form a good thermal and el ctrical intermetallic bond to the s "er by wet g. The window thus formed may be compared roughly to a thick wall of a soap bubble. A hole or bore is then pierced or drilled through the window 22. Where a plurality of a ertures are formed in the link, th window may be positioned across a penultimate aperture as well as an end aperture as shown in the drawings.

In another method of manufacture, an annular body such as a rivet, fabricated from low-melting temperature metal and having a pierced hole therethrough, is secured mechanically in the aperture by conventional methods. in the latter method, the electrical and thermal contact between he link and its alloy body are not as intimate as in the first method, giving somewhat different characteristics.

In both methods of manufacture, there are two longitudinal link portions and 14b adjoining the aperture interconnected by two narrow link portions 2% at the sides of the aperture, as shown in FIGS. 3 and 4, and the low-melting body is in contact both with the narrow link portions and with the adjoining longitudinal link portions l t-a and 14b.

When an over-current flows through the fuse link, where it is not of severity as to cause instantaneous heating and blowing of the link at one of the holes is or 13, the fuse link temperature rises progressively. The temperature varies from point to point along the fuse length, being lowest adjacent the terminal caps lltl and ill which function as heat sinks. The highest temperatures in the fuse link are developed where the cross-section of the link is the least and where the capability of the fuse to dissipate heat is the poorest. This hot spot occurs at constricted cross-sectional portions of the link as between the aperture 13 and the margins of the link ll l. The heat developed at this region is dissipated largely by conductiori toward the end caps. The heat at the constricted region at hole 13 in flowing toward end cap lil must traverse the region at hole 16 where additional heating and reduced cross-section of thermal conductivity path impede the flow of heat. Similarly, the heat which flows from the region of hole 18 toward end cap llll is impeded by the poorer thermal conductivity of the solder than the silver at aperture 26 and by the heat generated there.

When an overcurrent flows through the fuse link, localiZed high temperature thus develops in the region of the central aperture Heat that flows from the region of hole 18 and heat generated in link 34 at the constriction at hole 23 cause the temperature at the solder sass 22. to reach the melting point. The solder starts to flow toward the higher temperature portion of the fuse link, that is, from the lefthand extremity of aperture 25 toward aperture 118. As a result, hole 24 through the alloy window becomes progressively large The formation of a meniscus film is prevented by the bore 24. his may be likened to piercing a soap film. To some extent, the solder spreads laterally and develops a slightly increased thickness 22' at the edges of link at hole 2'9 (FIG. 6) but, as indicated in FIG. 5, the flow of solder is predominantly toward hole 13.

Interruption of the fuse ultimately occurs in response to prolonged moderate overloads at the region where hole 29 reduces the fuse link to a minimum cross-section, in the presence of the low-melting alloy. Interruption results from a number of factors. As the solder starts to melt, the total link cross-section for conduction of current along the link (MG. 6) is progressively reduced at hole 29, and as the conductive cross-section diminishes, the resistance and the heating rise disproportionately. Also, the thermal capacity of the material of the link in the region 2tla2tla of the fuse link 14 is reduced; and this means that as heat continues to flow from the region of central aperture 13, the temperature rise in the region of the solder is accelerated. With progressively increasing temperatures, the solder tends to alloy with the silver itself and the initially constricted region Ztla of the solder link gradually becomes an alloy having greatly increased resistivity. This further accelerates the melting of the solder, the migration of the molten solder, the reduction in cross-section of the link in the region of fuse-link constriction Ztla (PEG. 6) and the alloying process that increases resistivity locally.

The eflicacy of the improved fuse link in reducing linkto-link variation in response time may be seen from the following table which summarizes the data obtained when silver links with solder windows, pierced and unpierced, were subjected to a current of 100 amps. These links were essentially alike and therefore may be presumed to have the same hold-in current rating.

Blowing Time in Seconds Low melting temperature alloy: not picrccd Low melting temperature alloy: pierced for the unpierced-window links is from 162 to 505 secends, tiat is a range of 343 seconds about an average of approximately 480 seconds. This may be compared with a range for the pierced-window links of 147 to 243 secends, that is, a range of 96 seconds about an average of about 1%) seconds.

The improved consistency thus gained through the use the pierced-window link is of obvious importance. The l orter average blowing time of the pierced-window links represents a demonstration of improved trigger actions. 'ims means that the improved fuses will hold in indefinitely for rated current levels, and yet, after a limited e a' time the improved fuses will clear the circuit. Some delay time is desirable so that momentary overloads (commonly caused by the starting of motors and inrush currents of incandescent lamp loads) will not cause blowing of the fuse. Nevertheless, for abnormally prolonged overload curents the fuse characteristic should be such as to clear the circuit without protracted heating of the circuit wiring of the protected load or of the fuse itself. in the last-mentioned consideration, prolonged heating of the fuse by sustained overload current tends to overheat and weaken the cartridge. Upon blowing of the fuse, the arc then break through, with possibly serious consequences.

The present fuse holds in almost indefinitely while carrying lld% of rated current yet at of rated current it blows much faster than heretofore known fuses and performs with greater consistency (from one fuse to another) than known fuses of this type. Where some delay fuses may take an hour or more to blow at 135% of rating, fuses of the present construction can be made to blow within 5 to 10 minutes at 135% of rated current.

The hole 1% affords a constriction where blowing occurs almost instantly in respose to severe overloads. The pierced window provides for delay characteristics, and when the fuse blows in response to prolonged moderate overload, blowing always occurs at the window region. Accordingly, the fuse described is a dual element fuse,

raving both instant and delay response characteristics.

It is contemplated that the link lid may have additional apertures, similar to l6, 13, formed between the windowhlled aperture 2% and the adjacent end cap 11 when moditied characteristics are desired.

It will be apparent that the fuse described above, while being the presently preferred embodiment of the invention, is subject to a wide latitude of modification and varied applications by those skilled in the art, so that this "vention should be construed broadly in accordance with its full spirit and scope.

What is claimed is:

1. In an electric fuse, a fuse link including a fusible metal strip having a plurality of longitudinally spaced regions of reduced cross-section, one of said regions being formed by an aperture in the strip that defines two narrow marginal strip portions that interconnect adjoining longitudinal portions of the strip, a body of lower-melting-temper ature fusible metal secured to said metal strip at said aperture and in contact with said narrow strip portions with said adjoining lon itudinal portions of the strip, said body having an opening therethrough transverse to the strip but largely filling said aperture, said body being adapted to we; the metal strip and to flow along said metal strip when melted, said opening in said body preventing formation of a meniscus film of said lowcr-melting-temperature metal when melted in response to an overcurreht condition in said link.

2. in an electric fuse, a fuse link in accordance with claim 1 wherein said metal strip is silver and wherein said body is a relatively low-ternperature-melting alloy.

3. A cartridge fuse including a tube of insulating material, metal end caps on said tube providing electrical terminals and constituting heat sinks, and a fuse link, said fuse link including a fusible metal strip having a plurality of longitudinally spaced regions of reduced cross-section,

3,11 53 one of said regions being formed by an aperture in the strip that defines two narrow marginal strip portions that interconnect adjoining longitudinal portions of the strip,

a body of loweranelting-teinperatu re fusible metal secured to said metal strip at 5 id apenture and in Contact with said narrow strip portions and with said adjoining longitudinal portions of the strip, said body largely filling said aperture but having opening theretln'ougli tran verse to the strip, said body being adapted to Wet the metal strip and to flow along said metal strip when melted, said opening in said body preventing formation of a meniscus film of said lower melting temperature metal ,seo

when melted in response to an 'overourrent condition in said link, said link having at least one of said regions of reduced cross-section interposed between said body and at least one of said end caps.

References tilted in the file of this patent UNITED STATES PATENTS 1,473,284 Feldliamp NOV. 6, 1923 2,592,399 Edsall et a1 Apr. 8, 1952 2,832,868 Kozacka Apr. 29, 1958 2,866,037 Stewart Dec. 23, 1958

Claims

1. IN AN ELECTRIC FUSE, A FUSE LINK INCLUDING A FUSIBLE METAL STRIP HAVING A PLURALITY OF LONGITUDINALLY SPACED REGIONS OF REDUCED CROSS-SECTION, ONE OF SAID REGIONS BEING FORMED BY AN APERTURE IN THE STRIP THAT DEFINES TWO NARROW MARGINAL STRIP PORTIONS THAT INTERCONNECT ADJOINING LONGITUDINAL PORTIONS OF THE STRIP, A BODY OF LOWER-MELTING-TEMPERATURE FUSIBLE METAL SECURED TO SAID METAL STRIP AT SAID APERTURE AND IN CONTACT WITH SAID NARROW STRIP PORTIONS AND WITH SAID ADJOINING LONGITUDINAL PORTIONS OF THE STRIP, SAID BODY HAVING AN OPENING THERETHROUGH TRANSVERSE TO THE STRIP BUT LARGELY FILLING SAID APERTURE, SAID BODY BEING ADAPTED TO WET THE METAL STRIP AND TO FLOW ALONG SAID METAL STRIP WHEN MELTED, SAID OPENING IN SAID BODY PREVENTING FORMATION OF A MENISCUS FILM OF SAID LOWER-MELTING-TEMPERATURE METAL WHEN MELTED IN RESPONSE TO AN OVERCURRENT CONDITION IN SAID LINK.